One example of a wireless communications network using contention-based transmission resources of the same frequency is the standardized IEEE 802.11 Wireless LAN, WLAN. Here, a Basic Serving Set, BSS, is regarded the basic building block of the wireless communications network. The BSS comprise an Access Point, AP, and a number of stations, STAs, located within a certain coverage area or cell being served by the AP. Within a BSS, the transmission between the AP and the STAs is typically performed in a distributed manner. This means that before a transmission, a STA first performs a sensing of the transmission medium for a specific period of time, e.g. a Clear Channel Assessment, CCA. If the transmission medium is deemed idle, e.g. received signal power is below a threshold, then access is assigned to this STA for transmission. On the other hand, if the transmission medium is deemed occupied, e.g. received signal power is above the threshold, the STA typically has to wait a random back-off period and then again check whether the transmission medium is idle or occupied. For example, according to the current standard, the threshold is −82 dBm. The random back-off period provides a collision avoidance mechanism for multiple STAs that wish to transmit in the same BSS. In this case, the above contention-based channel access is commonly referred to as a distributed coordination function, DCF, in the IEEE 802.11 WLAN standard.
However, in many cases, there still exists STAs or APs that do not hear each other, e.g. the received signal power is too low, and will hence attempt to send their data simultaneously causing collisions at the receiver and hence data packet loss. To avoid this type of collisions, also commonly referred to as the hidden node problem, a medium access protocol comprising Request-to-Send, RTS, transmissions and Clear-to-Send, CTS, transmission has been proposed in the IEEE 802.11 WLAN standard. This RTS/CTS medium access protocol is illustrated in FIG. 1.
According to the RTS/CTS medium access protocol in FIG. 1, a transmitter, TX, in a BSS having data to transmit, e.g. a STA or an AP, will first send an RTS message to the intended receiver, RX, of the data in the BSS, e.g. an AP or a STA, in a first time slot. In response to the RTS message, the RX sends a Clear-to-Send, CTS, message back to the TX in a subsequent time slot. Upon receiving the CTS message, the TX may transmit its data to the RX in a following time slot, which data transmission then may be acknowledged or not by the RX. The RTS/CTS message may follow a Distributed Inter-Frame Space, DIFS, or Short Inter-Frame Space, SIFS, of the time slots. All other STAs or APs in the BSS that are able to receive or hear the RTS message from the TX will respond by setting their Network Allocation Vector, NAV, to defer its transmissions. This may be performed based on a duration field encoded in the RTS message which may indicate for how long time the NAV should be set for. The CTS message also comprises similar timing information to set the NAV, whereby all other STAs or APs in the BSS that receives or hears the CTS message will respond by doing so. The main purpose of the RTS/CTS message exchange, as described above with reference to FIG. 1, is to avoid collisions in a BSS, i.e. avoid multiple data packets at the same time being addressed to the same receiver over the same subcarriers.
Furthermore, to even further avoid collisions and/or interference in wireless communications networks comprising more than one BSS, different frequencies, subcarriers or channels, should be assigned to neighbouring or nearby BSSs. However, in dense deployment scenarios, it is likely that frequencies or channels will be reused even for neighbouring or nearby BSSs. In this case, resulting collisions and/or co-channel interference between the BSSs is expected to compromise the performance or Quality-of-Service, QoS, offered to the STAs by the BSSs. In particular, STAs that are located within an overlapping coverage area of the BSSs may, due to relatively strong interference, be more severely affected. In the IEEE 802.11 WLAN standard, this is commonly referred to as having Overlapping Basic Service Sets, OBSSs.
Although the RTS/CTS medium access protocol as described above with reference to FIG. 1 is useful, it will also affect other STAs or APs in other BSSs in case the BSS forms a part of an OBSS. If the OBSS is dense or closely spaced, these other STAs or APs will also set their NAV in the same way as the STAs and APs in the BSS even though they are operated independently of each other.
In such dense environments, the RTS/CTS medium access protocol may cause problems for the other STAs and APs in the other BSSs of the OBSS. Once a TX sends out an RTS message to an RX in a BSS, nearby STAs and APs that receives or hears the RTS message will consequently set their NAV. This will happen even if a STA or AP is not in the BSS, but close enough to receive or hear the RTS/CTS message exchange. The RTS/CTS messages are usually transmitted with the lowest Modulation and Coding Scheme, MCS.
However, in some circumstances, data transmissions from other STAs or APs in the OBSS will not interfere with the data transmission following the CTS message in the BSS. Hence, by blocking such data transmissions in the above described manner, the spatial reuse potential of the wireless communication network is reduced, especially in dense deployment environments.